Adhesive compositions

ABSTRACT

There is provided an adhesive composition comprising (a) a polyisocyanate component comprising (i) monomeric 4,4′ methylene diphenyl diisocyanate and (ii) an isocyanate functional prepolymer that is the reaction product of a prepolymer reactant mixture comprising (A) monomeric 4,4′ methylene diphenyl diisocyanate, and (B) one or more polyol; and (b) a polyol component comprising one or more polyol. Also provided is a method of bonding two or more substrates comprising bringing such an adhesive composition into contact with both of said two or more substrates. Further provided is a bonded assembly comprising two or more substrates bonded together by such an adhesive composition.

The following is the background of the invention.

A useful class of adhesive compositions are curable polyurethaneadhesive compositions. A useful category of such compositions aresolventless two-component compositions.

In such two-component compositions, one component is a polyisocyanatecomponent and the other component is a component that contains one ormore compounds with plural active hydrogens. When it is desired to usethe two-component composition as an adhesive, the two components aremixed together, and the mixture is brought into contact with two or moresubstrates.

When the two components of the adhesive composition are first mixedtogether, it is desirable that the resulting mixture is a fluid withviscosity at 25° C. low enough so that the fluid may easily be appliedto a substrate. Usually, soon after a layer of the fluid has beenapplied to a substrate, a second substrate is brought into contact withthat layer to form an assembly. When that assembly is first made, priorto any curing reaction taking place, the adhesive composition does notprovide any adhesive bond between the substrates. It is desired that theadhesive composition undergoes a curing reaction after formation of theassembly and forms an adhesive bond between the substrates to form abonded assembly. It is desirable that the adhesive composition showsgood bond strength development. That is, it is desirable that, when abonded assembly is first formed, the strength of the bond increasesrapidly. It is desirable that the bond have good strength at 2 to 8hours after formation of the bonded assembly. Good bond strengthdevelopment is important to converters (i.e., those who manufacturebonded assemblies) because good bond strength development allows theconverters to subject the bonded assembly to further manufacturing stepswithout the need to wait a long time for the adhesive composition tocure.

Some two-component adhesive compositions contain one or more monomericaromatic polyisocyanate compound and one or more polyol. Suchcompositions are capable of undergoing chemical reactions that formurethane polymers, which are often useful curing reactions. Themonomeric aromatic polyisocyanate may eventually react to form polymersand/or crosslinks that strengthen the adhesive bond.

These adhesive compositions are sometimes used to hold together two ormore substrates to form a bonded assembly that is part or all of acontainer that contains food. Even though a curing reaction has takenplace, some monomeric polyisocyanate compound may be present in thebonded assembly. The presence of monomeric polyisocyanate compound isconsidered to be undesirable. One reason for the undesirability ofmonomeric polyisocyanate compounds is that they are considered to beboth toxic and reactive. Another reason for the undesirability ofmonomeric polyisocyanate compounds is that they are capable of reactingwith water to form amines, especially under acidic conditions. Suchamines are considered to be undesirable. Among such amines, primaryaromatic amines (PAAs) are considered especially undesirable. When acontainer contains food, it is desired that contact between thecontainer and the food does not result in a situation in whichappreciable amounts of any PAAs are present in the food.

The capacity of a bonded assembly to contribute to the presence of PAAsin food is generally tested by exposing the bonded assembly to diluteacetic acid for a time. The dilute acetic acid acts as a “simulant”(i.e., it simulates the action of food). During that time, PAAs that maybe present in the bonded assembly may migrate into the simulant. Alsoduring that time, any monomeric aromatic isocyanate compounds in thematerial under test may migrate into the simulant and react with thesimulant to form PAA. Subsequently, the simulant may be analyzed for thetotal concentration of all PAAs. That concentration is herein called the“PAA level.”

It is desirable that a bonded assembly has low PAA level.

In the past, it was common for a bonded assembly that was made using atwo-component polyurethane adhesive to have an appreciable amount ofmonomeric aromatic polyisocyanate present, even after the formation ofthe bonded assembly and the curing reaction of the adhesive wereconsidered complete. Such a bonded assembly had a high PAA level.

In the past, one approach to providing a bonded assembly with a low PAAlevel was to make a bonded assembly and then place that bonded assemblyin storage prior to using the bonded assembly to make the package thatwill eventually contain food. The concentration of monomericpolyisocyanate will normally decline while the bonded assembly is instorage. It is thought that water in the atmosphere diffuses into theadhesive and reacts with isocyanate groups. While some of thesereactions may result in formation of PAA, most of these reactions willresult in formation of urea-type compounds or other relatively innocuouscompounds. Therefore, as the isocyanate groups react, the amount ofmonomeric aromatic polyisocyanate decreases and the PAA level alsodecreases. In the past, storage times of 14 days or more have often beenrequired before the PAA level becomes acceptably low.

Another approach is taken by US 2010/0010156, which describes anadhesive that leads to bonded multilayer films with reduced proportionof primary aromatic amines. US 2010/0010156 describes a polyurethaneadhesive that contains at least one NCO-reactive polyurethane prepolymerand/or polyisocyanates, wherein the polyurethane adhesive contains a lowmolecular weight compound which contains at least one group reactivewith primary amino groups selected from epoxide groups, (meth)acrylicgroups or carboxylic anhydride groups.

It is desired to provide a solventless two-component adhesivecomposition that has (in a relatively short time after formation of thebonded assembly) a desirably low level of PAAs. It is also desirable forthe adhesive composition to have desirably low viscosity when the twocomponents are first brought together; and to develop bond strengthdesirably quickly.

The following is a statement of the invention.

In a first aspect of the present invention, there is provided anadhesive composition comprising

-   -   (a) a polyisocyanate component comprising        -   (i) monomeric 4,4′ methylene diphenyl diisocyanate and        -   (ii) an isocyanate functional prepolymer that is the            reaction product of a prepolymer reactant mixture comprising            -   (A) monomeric 4,4′ methylene diphenyl diisocyanate, and            -   (B) one or more polyol comprising:                -   (B1) one or more triol having average molecular                    weight of 1,000 or less, and                -   (B2) one or more triol having average molecular                    weight greater than 1,000;            -   wherein the amount of monomeric 2,2′ methylene diphenyl                diisocyanate in said prepolymer reactant mixture is 1%                or less by weight based on the weight of said prepolymer                reactant mixture;    -   wherein the amount of monomeric 2,2′ methylene diphenyl        diisocyanate in said polyisocyanate component is 1% or less by        weight based on the weight of said polyisocyanate component;    -   and    -   (b) a polyol component comprising one or more polyol.

In a second aspect of the present invention, there is provided a methodof bonding two or more substrates comprising bringing the adhesivecomposition of the first aspect of the present invention into contactwith both of said two or more substrates.

In a third aspect of the present invention, there is provided a bondedassembly comprising two or more substrates bonded together by theadhesive composition of the first aspect of the present invention.

The following is a detailed description of the invention.

As used herein, the following terms have the designated definitions,unless the context clearly indicates otherwise.

As used herein, a compound is “volatile” if it has a boiling point atone atmosphere pressure of 100° C. or lower. As used herein, a “solvent”is a volatile compound that is liquid at one atmosphere pressure over atemperature range that includes 15° C. to 20° C. A composition isconsidered herein to be “solventless” if that composition contains atotal of all solvents in the amount of 2% or less by weight, based onthe total weight of the composition. As used herein, compositions havingsolvent in the amount of a certain percentage “or less” includescompositions with zero percent solvent. The “total solids” of acomposition is considered herein to be the amount of compounds in thecomposition that are not volatile, by weight based on the total weightof the composition.

As used herein, a polyisocyanate compound (also known herein as “apolyisocyanate”) is a compound that contains two or more isocyanategroups. Polyisocyanate compounds may be monomeric or non-monomeric. Anaromatic polyisocyanate is a polyisocyanate that contains one or morearomatic ring. A polyisocyanate that contains no aromatic rings is anon-aromatic polyisocyanate. As used herein, a “polyisocyanatecomponent” is either a polyisocyanate compound or else is a mixture ofcompounds, where one or more compound in the mixture is a polyisocyanatecompound.

As used herein, a “monomeric polyisocyanate” is a compound that has twoor more isocyanate groups, that has molecular weight of less than 1,000,and that has no urethane linkage and no urea linkage. Any polyisocyanatethat is not a monomeric polyisocyanate is a non-monomericpolyisocyanate.

As used herein, “MDI” is methylene diphenyl diisocyanate, also calleddiphenylmethane diisocyanate. MDI exists as one of three isomers (4,4′MDI, 2,4′ MDI, and 2,2′ MDI) or as a mixture of two or more of theseisomers. As used herein, unless specifically stated otherwise, “MDI”does not refer to polymeric MDI (sometimes called PMDI). Polymeric MDIis a compound that has a chain of three or more benzene rings connectedto each other by methylene bridges, with an isocyanate group attached toeach benzene ring.

A compound with plural active hydrogens is an organic compound that hastwo or more active-hydrogen groups. An active-hydrogen group is a groupthat has a hydrogen that is connected to an oxygen or a nitrogen. Acompound with two or more hydroxyl groups is a polyol. A polyol withexactly two hydroxyl compounds is a diol. A polyol with exactly threehydroxyl groups is a triol. A compound with two or more amine groups isa polyamine. Some compounds with plural active hydrogens have one ormore amine group and one or more hydroxyl group.

A compound that contains two or more ether linkages in the same linearchain of atoms is known herein as a polyether. A compound that containstwo or more ester linkages in the same linear chain of atoms is knownherein as a polyester. A compound that is a polyester and a polyol is apolyester polyol, and a compound that is a polyether and a polyol is apolyether polyol.

It is sometimes useful to characterize a compound by the number ofaliphatic carbon atoms it contains. As used herein, an aliphatic carbonatom is a carbon atom that belongs to one of the following types: (i) acarbon atom that has no bond to any atom other than other carbon atomsor hydrogen atoms; and (ii) a carbon atom that has one single bond toone heteroatom selected from oxygen, nitrogen, and sulfur and thatotherwise has no bond to any atom other than hydrogen atoms or carbonatoms of category (i).

An alkyl compound that has two or more hydroxyl groups attached to it isknown herein as an alkyl polyol; an alkyl polyol has no substituentgroups other than hydroxyl groups. All of the atoms in an alkyl polyolare carbon and hydrogen except for the oxygen atoms in the hydroxylgroups.

Some non-monomeric polyisocyanates are reaction products of one or morepolyisocyanate with one or more compound with plural active hydrogencompounds, where such reaction products have two or more un-reactedisocyanate groups. Such non-monomeric polyisocyanates may be, forexample, the reaction products of one or more polyisocyanate with one ormore polyol or the reaction products of one or more polyisocyanate withone or more polyamine or a mixture thereof. A polyisocyanate that is areaction product of one or more polyisocyanate with one or more compoundwith plural active hydrogen compounds and that has molecular weight of1,000 or more is known herein as a prepolymer. Because the prepolymer isa polyisocyanate, it is known herein as an isocyanate functionalprepolymer.

A useful method of characterizing the amount of isocyanate groups in acomposition is “% NCO,” which is the total weight of all isocyanategroups present in the composition, divided by the total weight of thecomposition, expressed as a percentage.

As used herein, a plasticizer is a compound that has no isocyanate groupand that has no active-hydrogen group. To qualify as a plasticizer, acompound must also be soluble at 25° C. in an adhesive composition ofthe present invention.

As used herein, a “polyurethane” is a compound that contains a linearchain of atoms that contains three or more linkages selected fromurethane linkages and urea linkages.

A composition is said herein to “cure” as chemical reactions take placethat desirably effect an increase in the molecular weight of thecomposition and/or effect crosslinking of the composition so as toimprove the properties of the composition. Such chemical reactions areknown as “curing reactions.” The composition is said to be “cured” whensuch reactions are complete or when the curing reactions have progressedsufficiently far that the properties of the composition are useful andare not appreciably changing over time. A composition that is capable ofundergoing one or more curing reaction is a curable composition.

As used herein, when a composition is said to have “little or no” amountof an ingredient, it is meant that the amount of that ingredient iseither zero or is 0.1% by weight or less, based on the weight of thecomposition.

When it is said herein that a certain material is “not soluble” in theliquid portion of an adhesive composition, it is meant that there is notemperature in the range of 20° C. to 50° C. at which that material canbe dissolved in the liquid portion of that adhesive composition in theamount of 1% or more by weight, based on the weight of the liquidportion of the adhesive composition.

A polymer film is a structure that is made of a polymer or mixture ofpolymers and that is 0.5 mm or less in one dimension and is 1 cm or morein both of the other two dimensions. The composition of a polymer filmis 80% or more by weight one or more polymer, based on the weight of thefilm.

When a quantity is described herein as “X to Y,” it is meant that thequantity is a number that is greater than or equal to X and also is lessthan or equal to Y. When a ratio is described herein as “R:1 orgreater,” it is meant that the ratio is S:1, where S is a number greaterthan or equal to R. Similarly, when a ratio is described herein as “T:1or less,” it is meant that the ratio is U:1, where U is a number lessthan or equal to T.

The amount of isocyanate any composition that contains both isocyanategroups and compounds with active hydrogens may usefully be characterizedby a quantity herein called “isocyanate index,” which is a calculatedquantity, based on the ingredients and amounts that are present in thecomposition. This value is the quotient obtained by dividing the molesof isocyanate groups by the moles of active hydrogens.

The composition of the present invention contains a polyisocyanatecomponent, which contains 4,4′ MDI and an isocyanate functionalprepolymer. The polyisocyanate component preferably also contains 2,4′MDI. The polyisocyanate component of the present invention eithercontains no 2,2′ MDI or, if some 2,2′ MDI is present, the amount of 2,2′MDI is, by weight based on the weight of the polyisocyanate component,1% or less. Preferably the amount of 2,2′ MDI in the polyisocyanatecomponent, by weight based on the weight of the polyisocyanatecomponent, is 0.5% or less; more preferably 0.2% or less; morepreferably 0.1% or less.

The polyisocyanate component of the present invention also contains oneor more isocyanate functional prepolymer. The reactant mixture that isused for making the isocyanate functional prepolymer is known herein asthe prepolymer reactant mixture. The prepolymer reactant mixturepreferably contains one or more monomeric polyisocyanate and one or morecompound that has plural active hydrogens. The amount of monomericpolyisocyanate in the prepolymer reactant mixture is preferably, byweight based on the weight of the prepolymer reactant mixture, 15% ormore; more preferably 25% or more. The amount of monomericpolyisocyanate in the prepolymer reactant mixture is, by weight based onthe weight of the prepolymer reactant mixture, 75% or less; or 60% orless.

Preferred prepolymer reactant mixtures have little or no non-aromaticmonomeric polyisocyanates. More preferred prepolymer reactant mixtureshave no non-aromatic monomeric polyisocyanates. More preferredprepolymer reactant mixtures have no monomeric polyisocyanate other than4,4′ MDI, 2,4′ MDI, 2,2′ MDI, and mixtures thereof.

The amount of compound having plural active hydrogens in the prepolymerreactant mixture is preferably, by weight based on the weight of theprepolymer reactant mixture, 25% or more; more preferably 40% or more.The amount of compound having plural active hydrogens in the prepolymerreactant mixture is preferably, by weight based on the weight of theprepolymer reactant mixture, 75% or less; more preferably 60% or less.

The prepolymer reactant mixture contains one or more polyol. Morepreferably, every compound with plural active hydrogens in theprepolymer reaction mixture is a polyol. The amount of triol in theprepolymer reactant mixture is preferably, by weight based on the totalweight all polyols in the prepolymer reactant mixture, 25% or more; morepreferably 40% or more. The amount of triol in the prepolymer reactantmixture is preferably, by weight based on the total weight all polyolsin the prepolymer reactant mixture, 75% or less; more preferably 60% orless.

The prepolymer reactant mixture preferably comprises one or more triolhaving average molecular weight greater than 1,000. More preferably, theprepolymer reactant mixture comprises one or more triol having averagemolecular weight greater than 2,500.

The prepolymer reactant mixture preferably additionally contains one ormore diol. Preferably, the prepolymer reactant mixture contains one ormore diol having average molecular weight of 1,000 or less. Preferably,prepolymer reactant mixture also contains one or more diol havingaverage molecular weight of greater than 1,000. A preferred prepolymerreactant mixture contains B1 (one or more triol having average molecularweight of 1,000 or less); and B2 (one or more triol having averagemolecular weight of more than 2,000). A more preferred prepolymerreactant mixture contains B1, B2, and B3 (one or more diol havingaverage molecular weight of 1,000 or less). A more preferred prepolymerreactant mixture contains B1, B2, B3, and B4 (one or more diol havingaverage molecular weight of more than 1,000). Preferably, each of B1,B2, B3, and B4 (if present) is either a polyether polyol or a polyesterpolyol. Preferably, B1 and B2 are both polyether triols.

Polyols may be usefully characterized by their “OH equivalent weight,”which is the molecular weight of the polyol divided by the number ofhydroxyl groups. A preferred prepolymer reactant mixture contains one ormore “low-ew” diol (i.e., a diol having OH equivalent weight of lessthan or equal to 200; one or more “med-ew” triol (i.e., a triol havingOH equivalent weight of 141 to 342), and one “high-ew” triol (i.e., atriol having OH equivalent weight of 1,500 or greater). A more preferredprepolymer reactant mixture contains one or more low-ew polyether diol,one or more med-ew polyether triol, and one or more high-ew polyethertriol.

Preferably, in the prepolymer reactant mixture, every compound havingplural active hydrogens is a polyol. Preferably, in the prepolymerreactant mixture, every compound that has one or more active hydrogensis a polyol.

The prepolymer reactant mixture contains monomeric 4,4′ MDI. Preferably,the prepolymer mixture also contains 2,4′ MDI. Preferably, the amount of2,2′ MDI in the prepolymer reactant mixture, by weight based on theweight of all polyisocyanate compounds in the prepolymer reactantmixture, is 0% to 1%; more preferably 0% to 0.5%. Preferably, the amountof 2,4′ MDI in the prepolymer reactant mixture, by weight based on theweight of all polyisocyanate compounds in the prepolymer reactantmixture, is 0% to 50%.

Preferably, the isocyanate index of the prepolymer reactant mixture is2.0 or greater; more preferably 2.5 or greater; more preferably 3.0 orgreater. Preferably, the isocyanate index of the prepolymer reactantmixture is 7 or less; more preferably 6 or less; more preferably 5 orless.

The polyisocyanate component of the composition of the present inventioncontains 4,4′ MDI and one or more isocyanate functional prepolymer. The% NCO of the polyisocyanate component is preferably 7% or higher; morepreferably 10% or higher. The % NCO of the polyisocyanate compound ispreferably 25% or lower; more preferably 17% or lower.

It is useful to characterize the amount of polyisocyanate in theadhesive composition. That amount is the sum of the weights of allpolyisocyanate compounds divided by the total weight of the adhesivecomposition, expressed as a percentage. The preferred amount ofpolyisocyanate in the adhesive composition of the present invention is10% or more by weight based on the weight of the adhesive composition.The more preferred amount of polyisocyanate in the adhesive compositionof the present invention is, by weight based on the weight of theadhesive composition, 20% or more; more preferred is 30% or more. Also,the preferred amount of polyisocyanate in the adhesive composition ofthe present invention is 90% or less by weight based on the weight ofthe adhesive composition; the more preferred amount of polyisocyanate inthe adhesive composition of the present invention is, by weight based onthe weight of the adhesive composition, 80% or less.

The preferred % NCO of the composition of the present invention is 4% ormore; more preferably 6% or more; more preferably 8% or more. Thepreferred % NCO of the composition of the present invention is 20% orless; more preferably 17% or less; more preferably 14% or less.

The adhesive composition of the present invention contains a polyolcomponent. Preferably, the polyol component contains one or more triolhaving molecular weight of 100 to 1,000. More preferably the polyolcomponent contains one or more triol having average molecular weight ofless than 400 and also contains one or more triol having averagemolecular weight more than 400.

The total amount of all triols in preferred polyol components, by weightbased on the total weight of all polyols present in the polyolcomponent, is 60% to 100%; more preferably 75% to 100%; more preferably90% to 100%; more preferably 100%.

Preferred polyol components include one or more polyol selected fromalkyl polyols, castor oil, polypropylene glycol triols, and mixturesthereof. When an alkyl triol is present, preferred alkyl triols have 3or more carbon atoms; more preferably 4 or more carbon atoms. When analkyl triol is present, preferred alkyl triols have 10 or fewer carbonatoms; more preferably 8 or fewer carbon atoms. When an alkyl triol ispresent, the most preferred alkyl polyol is trimethylol propane.Preferred polyol components contain two or more polypropylene glycoltriols, where one or more polypropylene glycol triol has averagemolecular weight of 400 or lower, and one or more polypropylene glycoltriol has average molecular weight above 400.

Preferred adhesive compositions contain little or no compound havingplural active hydrogens other than polyols. More preferred adhesivecompositions contain little or no compound having plural activehydrogens other than triols.

Compositions of the present invention preferably have isocyanate indexof 0.8 or higher; more preferably 0.9 or higher. Compositions of thepresent invention have isocyanate index of 1.8 or lower; more preferably1.7 or lower; more preferably 1.6 or lower.

The composition of the present invention is solventless. Preferredcompositions of the present invention contain solvent in the amount, byweight based on the weight of the composition, of 1% or less; morepreferably 0.5% or less; more preferably 0.2% or less. The total solidsof the composition of the present invention is preferably 98% or higher;more preferably 99% or higher; more preferably 99.5% or higher; morepreferably 99.8% or higher.

Preferably, every isocyanate compound in the composition of the presentinvention is contained in the polyisocyanate component. Preferably,every compound in the composition of the present invention that has oneor more active hydrogen is contained in the polyol component. Preferredadhesive compositions of the present invention have little or nonon-aromatic monomeric polyisocyanates. More preferred adhesivecompositions have no non-aromatic monomeric polyisocyanates. Morepreferred adhesive compositions have no monomeric polyisocyanate otherthan 4,4′ MDI, 2,4′ MDI, 2,2′ MDI, and mixtures thereof.

In the compositions of the present invention, preferred weight ratio ofpolyol component to isocyanate component is 0.1:1 or greater; morepreferably 0.2:1 or greater; more preferably 0.4:1 or greater. In thecompositions of the present invention, preferred weight ratio of polyolcomponent to isocyanate component is 2:1 or less; more preferably 1.2:1or less; more preferably 0.8:1 or less.

The adhesive composition of the present invention optionally containsone or more plasticizer. When a plasticizer is present, preferredplasticizers have 8 or more aliphatic carbon atoms. When a plasticizeris present, preferred amount of plasticizer is 0.5% to 15% by weightbased on the weight of the adhesive composition.

A catalyst such as, for example, a tertiary amine or a tin-basedcatalyst or a mixture thereof is optionally admixed with the ingredientsof the adhesive composition. In preferred embodiments, little or nocatalyst is present. In more preferred embodiments, no catalyst ispresent.

The adhesive composition may be formulated by admixing additionalconventional ingredients such as fillers, pigments, tackifiers,plasticizers, rheology modifiers, polymers (including, for example,thermoplastic resins other than those discussed herein above),dehydrating agents (including, for example, silanes), benzoyl chloride,other polyols (including, for example, fatty polyols), ultravioletindicators, etc. If such additional conventional ingredients are used,they are chosen and used with due regard to the reactivity of theisocyanate groups, which is desirably maintained. If such additionalconventional ingredients are used, they may be added, individually or inany combination, to the composition before the other ingredients, afterthe other ingredients, or during any stage or combination of stages ofthe formation of the adhesive composition.

In preferred embodiments, the adhesive composition of the presentinvention contains little or no compound that has one or more reactivegroup selected from epoxide groups, acrylic groups, methacrylic groups,and carboxylic anhydride groups. More preferred are embodiments in whichthe adhesive composition of the present invention contains no compoundthat has one or more reactive group selected from epoxide groups,acrylic groups, methacrylic groups, and carboxylic anhydride groups.

The curable adhesive composition of the present invention undergoes, oneor more chemical reaction that desirably effects an increase in themolecular weight of the composition and/or effects crosslinking of thecomposition so as to improve the properties of the composition.

In embodiments that involve application of the curable adhesivecomposition of the present invention to at least one substrate, theapplication is preferably effected by conventional means such as, forexample, spray applicator, bead applicator, nozzle, doctor blade,extrusion, or roll coater, to form a continuous or discontinuous film ofadhesive, as desired.

The composition may alternatively or additionally be applied to thesubstrate by hand, for example with a hand-held tool such as, forexample, a spatula, hand-held metering device, or other applicator. Thecomposition is preferably applied at a level, in grams per square meter,of 0.5 or greater; more preferably 1 or greater. The composition ispreferably applied at a level, in grams per square meter, of 5 or less;more preferably 3 or less.

In some embodiments, the curable adhesive composition of the presentinvention is used as an adhesive to bond a first substrate to at leastone subsequent substrate. In such embodiments, the adhesive compositionis applied to a first substrate, and, subsequently or simultaneously,the applied adhesive composition is contacted by at least one subsequentsubstrate to provide a bonded assembly.

In some of the embodiments in which the curable adhesive composition ofthe present invention is used as an adhesive to bond a first substrateto at least one subsequent substrate, the structure so formed isoptionally subjected to applied pressure such as by passing thestructure between rollers to effect increased contact of the substrateswith the curable adhesive composition. If such pressure is applied, itmay be applied for a relatively short duration or for a relatively longduration; and it may be constant, increasing, decreasing, or acombination thereof. If the structure is at temperature above roomtemperature, it is then cooled or allowed to cool. If pressure isapplied, cooling may take place during application of the pressure,after application of the pressure, or a combination thereof.

In preferred embodiments of the present invention, the adhesivecomposition of the present invention contains little or no solid polymerthat is not soluble in the liquid portion of the adhesive composition ofthe present invention. More preferred are adhesive compositions thatcontain no such polymer.

In preferred embodiments of the present invention, the adhesivecomposition of the present invention contains little or no solidmaterial of any composition that is not soluble in the liquid portion ofthe adhesive composition of the present invention. More preferred areadhesive compositions that contain no such solid material.

In preferred embodiments, the adhesive composition of the presentinvention contains little or no acrylic polymer. An acrylic polymer is apolymer in which 25% or more of the polymerized monomer units, by weightbased on the weight of the polymer, are selected from acrylic ormethacrylic acid, substituted and unsubstituted esters thereof, andsubstituted and unsubstituted amides thereof. More preferred areembodiments in which the adhesive composition of the present inventioncontains no acrylic polymer.

In preferred embodiments, the adhesive composition of the presentinvention contains little or no polymer that is not a polyurethane. Morepreferred are embodiments in which the adhesive composition of thepresent invention contains no polymer that is not a polyurethane.

In preferred embodiments, the adhesive composition of the presentinvention contains little or no compound that has a reactive group otherthan isocyanate and hydroxyl. More preferred are adhesive compositionsthat contain no compound that has a reactive group other thanisocyanate, hydroxyl, and amine. Still more preferred are adhesivecompositions that contain no compound that has a reactive group otherthan isocyanate and hydroxyl.

In preferred embodiments of the present invention, the curable adhesivecomposition of the present invention is applied to, or otherwisecontacted to, one or more substrates. Some suitable substrates include,for example, wood, metal, plastics, composite materials, fabric(including woven and non-woven), paper, and combinations thereof.Suitable substrates include, for example, natural wood, plywood, lauanwood, particle board, oriented strand board, rigid plastic, flexibleplastic, plastic films, plastic foams, and combinations thereof. If morethan one substrate is used, any combination of suitable substrates isalso suitable. In some embodiments, all the substrates will be made ofthe same material; in other embodiments, two or more different materialswill be used as substrates.

In preferred embodiments, one or more substrates is a polymer film. Morepreferred are embodiments in which the curable adhesive composition isused to bond together two polymer films. Preferred polymer films containorganic polymer. Preferred organic polymers are polyesters, polyolefins(including copolymers of olefins with other monomers), polyamides, andblends thereof. More preferred organic polymers are polyethyleneterephthalate, polyethylene, polypropylene, and nylon.

It is useful to consider embodiments herein called “ordinary cure”embodiments. In ordinary cure embodiments, two polymer films are bondedtogether to form a laminate, using a curable adhesive composition of thepresent invention. The curing reaction is allowed to progress to a pointin time (herein “T1”) at which the adhesive properties are acceptableand at which the adhesive properties do not change appreciably overtime. During a process of ordinary cure, it is useful to assess the PAAlevel as a function of time. Usually, after T1 is reached, the PAA leveldecreases over time as the laminate is stored. It is desirable that thelevel of PAA becomes acceptably low in as short a time as possible.

Some types of films allow diffusion of water more readily than othertypes of films. For some choices of film types, a laminate (prior to thepresent invention) might have taken 14 days or more, or even 20 days ofstorage after formation of the laminate before the PAA level wasacceptably low. It is contemplated that laminates made with the adhesivecomposition of the present invention will achieve acceptably low PAAlevels in shorter times than comparable previously-known adhesives.

A laminate that is made of two films may be made of two films that areidentical to each other or may be made of two films that are differentfrom each other.

A desirable characteristic of curable adhesive compositions is bondstrength development. When the moisture-reactive composition is used tobond two substrates together, bond strength development is assessed bymeasuring the bond strength prior to completion of the curing reaction.In particular, bond strength development is assessed by measuring thebond strength at 2 to 8 hours after formation of the bonded assembly.

The following is a discussion some desirable features of adhesivecompositions intended for making laminates. Prepolymers havingrelatively low content of monomeric polyisocyanate are useful increating adhesive compositions that produce lower levels of PAAs.However, it is often useful to also consider viscosity (at 23° C.) andthe adhesive working time when contemplating a two-component solventlessurethane adhesive. Relatively high level of monomeric polyisocyanateserves as a viscosity suppressant in solvent-free urethane prepolymers.Typically the viscosity of useful isocyanate terminated prepolymers withrelatively low content of monomeric polyisocyanate is greater than14,000 mPas at 23° C., as measured by Brookfield Viscometer model DVII+,with #27 spindle at 20 rpm. Useful prepolymers intended for laminatingthat have relatively high content of monomeric polyisocyanate typicallyexhibit relatively high mixed viscosities when combined with the polyolcomponent. The higher mixed viscosity limits the application line speedon a commercial solvent-free laminating machine. Potlife, or workingtime, is necessary to maintain the proper adhesive wet characteristicsduring the application process. An appropriate pot-life is consideredthe time it takes for the viscosity of properly mixed components todouble. An appropriate pot-life for two-component laminating adhesivesis about 16 minutes or greater at the application temperature. Theapplication temperature of the laminating adhesive should be greaterthan ambient (i.e., 23° C.) to enable appropriate temperature control,it is set between 40-50° C. for optimum machine control. In order toenable proper application characteristics, the mixed viscosity of thetwo components is preferably less than 1500 mPas at the applicationtemperature, more preferably at 1200 mPas or less.

It is to be understood that for purposes of the present examples thateach operation disclosed herein is performed at 25° C. unless otherwisespecified. “Ambient conditions” are approximately 23° C.

The following are examples of the present invention.

The following abbreviations are used:

Term: Definition Term: Definition Abbr. Abbreviation MW averagemolecular weight MDI methylene diphenyl EVA ethylene vinyl acetatediisocyanate copolymer DEG diethylene glycol pbw parts by weight OHew OHequivalent weight Sub1 Primary Substrate Ingr. Ingredient Sub2 SecondarySubstrate

The following ingredients were used:

Code Description I-A Standard Mixture of 4,4′ MDI, 2,4′ MDI, and 2,2′MDI I-B Mixture of 4,4′ MDI, 2,4′ MDI, and 2,2′ MDI with reduced levelof 2,2′ MDI I-C 98% 4,4′ MDI, 2% 2,4′ MDI (by weight) P-A adipatepolyester diol, OHew = 860-1125, based on mixed glycols P-B Polyethertriol, OHew = 1400-1900 P-C Polyether triol, OHew = 220-250 P-D DEGadipate polyester diol, OHew = 400-535 P-E Polyether diol, OHew =200-225 P-F Castor Oil P-G Polyether triol, OHew = 82-88 P-H Polyethertriol, OHew = 140-155 P-I trimethylol propane (TMP) P-M adhesionpromoter OPA 85% by weight solution of orthophosphoric acid in water Z-1di-isononyl ester of 1,2 cyclohexane dicarboxylic acid

The following film substrates were used.

Code Composition of film Thickness (micrometers) PET polyester 12 PEpolyethylene 80 coexPP coextruded polypropylene 20 metPP metallizedcoextruded polypropylene 20 Nylon nylon 15 PE-E vinyl acetate modifiedpolyethylene 50 Foil prelaminated aluminum foil

EXAMPLE 1 Preparation of Adhesive Composition

Materials for the prepolymer were as follows:

Ingredient: I-C I-B P-A P-B P-C P-D P-E pbw: 20.7 30.9 10.1 21.9 1.8 3.511.1

Preparation of prepolymer (isocyanate component): Nitrogen sweep of 0.5m³/hr was maintained. Reactor temperature was 35° C. I-A and I-B wereadded, and the agitator was started and run at approximately 62 rpm. Oneat a time, polyols were added in order as follows: P-A, P-B, P-C, P-D,and P-E. The reactor was held at 80° C. to 84° C., using cooling ifnecessary, for 2.5 hours. Product was placed under a nitrogen blanket ina closed container.

Materials for the polyol component were as follows:

Ingredient: Castor Oil P-F P-G TMP pbw: 61 15 20 4

Preparation of the polyol component: Nitrogen sweep as above wasmaintained in the reactor. Castor oil was placed into the reactor, andthe agitator was turned on an set to 60-65 rpm. P-F, P-G, and TMP wereadded. The reactor was set to 80° C. to 84° C. The reactor was sealedand the nitrogen sweep was shut off. The reactor was held at 80° C. to84° C. for 60 minutes. Products were placed under a nitrogen blanket ina closed container.

Adhesive composition was prepared by mixing 100 pbw of isocyanatecomponent with 45 pbw of the polyol component.

EXAMPLE 2C Preparation of Comparative Adhesive

The polyisocyanate component was prepared as in Example 1, using thefollowing ingredients:

Ingredient: I-C I-A P-A P-B P-C P-D P-E pbw: 20.7 30.9 10.1 21.9 1.8 3.511.1

The polyol component was prepared as in Example 1, using the followingingredients:

Ingredient: Castor Oil P-F P-G TMP pbw: 61 15 20 4

The polyisocyanate component and the polyol component were mixed as inExample 1.

EXAMPLE 3 Laminates were Prepared as Follows

Immediately after combining the polyisocyanate component with the polyolcomponent, the adhesive (at ambient temperature) was then fed onPolytype™ roll coater (metered rolls set at 40° C. to 45° C.). Mixedadhesive was then applied to in line corona treated primary web at thecoverage of 1.6 to 2.2 grams per square meter or 1 to 1.2 lbs. per ream,and then adhesive coated primary web was mated to secondary web and thenthis laminated film was nipped and wound on the finish roll on Polytype™laminator. Preparation of laminate for bond data: After appropriatecuring time, 15 mm wide strip was cut from the finished roll and bonddata was generated on Instron™ tensile tester using separation speed of100 mm/minute (4 inches/minute). Results are reported as grams of forcerequired to peel the laminate apart (abbreviated as “f(g)”).

EXAMPLE 4 Testing

Laminates for PAA testing were prepared as described above. Primarysubstrate was nylon, and the secondary substrate was EVA modified PE.EVA modified PE is chosen for PAA testing because EVA modified PE ismore porous to undesirable migratory species than PE without VAmodification. Laminates made with substrates other than EVA modified PEgenerally exhibit faster PAA decay than do laminates made with EVAmodified PE. Therefore, if a laminate is made with a certain adhesivecomposition, has at least one substrate that is EVA modified PE, andshows acceptably fast PAA decay, then other laminates made with thatsame certain adhesive composition are also likely to show PAA decay thatis at least fast enough to be acceptable.

PAA level was measured as follows. After cure on the laboratory bench atapproximately 23° C. and 50% relative humidity for 7 days, PAA wasmeasured. Laminates were exposed to simulant (3% acetic acid in water byweight) for 2 hours at 70° C. PAAs in the simulant were derivatized bydiazotization so that the concentration of PAAs could be determinedcolorimetrically. The concentration of PAAs is reported as anilinehydrochloride equivalents, as micrograms of aniline hydrochloride per100 ml of simulant. The procedure was as follows. All solutions areaqueous. 100 ml of simulant was mixed with 12.5 ml of hydrochloric acidsolution (1N) and 2.5 ml of sodium nitrite solution (0.5 g per 100 ml ofsolution) and was allowed to react for 10 minutes. 5 ml of ammoniumsulfamate (2.5 g per 100 ml of solution) was added and allowed to reactfor 10 minutes. 5 ml of coupling reagent (1 g ofN-(1-naphtyl)-ethylenediamine dihydrochloride per 100 g of solution) wasadded and allowed to react for 120 minutes. The solution wasconcentrated by elution through an ODS solid phase extraction column,and the extinction coefficient was measured at 550 nm. To determine theconcentration of PAA, the extinction coefficient was compared with theextinction coefficients of blank samples and a variety of samples withknown concentration of aniline hydrochloride.

Tensile tests were performed using ASTM D1876 (ASTM International, WestConshohocken, Pa., USA). Separation speed was 100 mm/min, and samplewidth was 15 mm. Bond data reported is the average of two specimenstested under appropriate condition. The force required to separate thesubstrates at 3 to 8 hours is considered to be a measure of the “bondstrength development” of the laminate. “FF” means that the substratefailed; that is, the layer of adhesive composition and the bond betweenadhesive composition and substrate were stronger than the substrateitself.

Results of PAA Analysis:

Laminate PAA level⁽²⁾ Comparative 2C 3.8 Example 1 0.49 Note (2):microgram Aniline HCl/100 ml of simulant

The Example of the present invention showed far less PAA than thecomparative example.

Results of tensile testing was as follows. Times shown are the length ofthe interval from formation of the laminate until testing.

bond strength g/15 mm Sub1 Sub2 Example No. 3 hr 4 hr 5 hr 6 hr PET PE 1150 261 323 432FF coexPP metPP 1 27 96 137 140 nylon PE 1 67 215 386FF392FF

For each type of laminate, the Example 1 of the present invention showedgood bond strength development.

EXAMPLE 5 Preparation of Adhesive Composition

Prepolymer was made as in Example 1. Materials for the prepolymer wereas follows:

Ingredient I-C I-B P-D P-A P-E P-C P-B Z-1 pbw: 18.4 28.8 3.7 10.1 11.11.8 21.9 4.2

The polyol component was made as in Example 1. Materials for the polyolcomponent were as follows:

Ingredient: P-C P-H P-G OPA pbw: 66.7 21.5 11.6 0.2

The adhesive composition was prepared by mixing 100 pbw of isocyanatecomponent with 35 pbw of the polyol component.

EXAMPLE 6C Preparation of Comparative Adhesive Composition

Prepolymer was made as in Example 1. Materials for the prepolymer wereas follows:

Ingredient I-A I-C P-D P-A P-E P-C P-B Z-1 pbw: 28.8 18.4 3.46 10.1 11.12.07 21.9 4.15

The polyol component was made as in Example 1. Materials for the polyolcomponent were as follows:

Ingredient: P-H P-M pbw: 99 1

The adhesive composition was prepared by mixing 100 pbw of isocyanatecomponent with 35 pbw of the polyol component.

EXAMPLE 7 Measurements of Example 5 and Comparative Example 6C

PAA results were as follows:

Laminate PAA⁽²⁾ Example 5 0.36 Comparative Example 6C 1.35 Note (2):microgram Aniline HCl/100 ml of simulant

Example 5 showed much lower PAA than Comparative Example 6C. It isconsidered that the difference in PAA between the two samples is due tothe difference in the MDI isomers that were used in the two differentpolyisocyanate components.

Additionally, Sample 5 had acceptable bond strength development.Laminates were made and tested, using Example 5 as the adhesive. Afterappropriate curing time, 2.54 cm (one inch) wide strip was cut from thefinished roll and bond data was generated on Instron™ tensile testerusing separation speed of 25.4 cm/minute (10 inches/minute). Results arereported as grams of force required to peel the laminate apart(abbreviated as “f(g)”). Failure modes are reported as follows:

Sub=substrate failure

Coh=cohesive failure; adhesive composition remains on both substrates

At=adhesive transfer

Results shown were measured at the time intervals shown after thelaminate was made and then stored at approximately 23° C. Units aregrams of force to peel the sample that was 2.54 cm (1 inch) in width.

Peel Force at Various Time Intervals Sub1 Sub2 3 hrs 8 hrs 24 hrs 7 days14 days foil PE f(g)  61  554 1234 731 638 mode Coh Coh Sub At At PET PEf(g) 207 1102 2771 2546  2712  mode Coh Sub Sub Sub Sub

Both laminates showed good bond strength development.

We claim:
 1. An adhesive composition comprising (a) a polyisocyanatecomponent comprising (i) monomeric 4,4′ methylene diphenyl diisocyanateand (ii) an isocyanate functional prepolymer that is the reactionproduct of a prepolymer reactant mixture comprising (A) monomeric 4,4′methylene diphenyl diisocyanate, and (B) one or more polyol comprising,(B1) one or more triol having average molecular weight of 1,000 or less,and (B2) one or more triol having average molecular weight greater than1,000; wherein the amount of monomeric 2,2′ methylene diphenyldiisocyanate in said prepolymer reactant mixture is 0.5% or less byweight based on the weight of all polyisocyanate compounds in saidprepolymer reactant mixture; wherein the amount of monomeric 2,2′methylene diphenyl diisocyanate in said polyisocyanate component is 1%or less by weight based on the weight of said polyisocyanate component;and (b) a polyol component comprising one or more polyol.
 2. Theadhesive composition of claim 1, wherein said polyol component comprisesone or more triol having average molecular weight of 100 to 1,000. 3.The adhesive composition of claim 1, wherein said prepolymer reactantmixture further comprises 2,4′ methylene diphenyl diisocyanate.
 4. Theadhesive composition of claim 1, wherein every compound having pluralactive hydrogens in said prepolymer reactant mixture is a polyol.
 5. Theadhesive composition of claim 1, wherein the total weight of all triolsin said prepolymer reactant mixture is 25% to 75%, based on the totalweight of all polyols in said prepolymer reactant mixture.
 6. Theadhesive composition of claim 1, wherein said polyol component comprisesone or more triol having average molecular weight of less than 400 andone or more triol having average molecular weight of more than
 400. 7.The adhesive composition of claim 1, wherein the total weight of alltriols in said polyol component is, by weight based on the weight of allpolyols in said polyol component, 60% to 100%.
 8. A method of bondingtwo or more substrates comprising bringing the adhesive composition ofclaim 1 into contact with both of said two or more substrates.
 9. Abonded assembly comprising two or more substrates bonded together by theadhesive composition of claim 1.